Transcript of "Ej35767774"

1.
Reda S. Abdel Hameed et al. Int. Journal of Engineering Research and Application
ISSN : 2248-9622, Vol. 3, Issue 5, Sep-Oct 2013, pp.767-774
RESEARCH ARTICLE
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OPEN ACCESS
Kinetics and Mechanism of Base-Catalyzed Oxidation of 1, 4, 7,
10-Tetrazacyclododecane-1-Carbaldehyde with Hexacyanoferrate
(III)
Abd-Alhakeem H. Abu-Nawwas (A, B)* and Reda S. Abdel Hameed (A, C).
a
Chemistry Department, College of Science, Hail University, Hail 1560, KSA.
School of Chemistry, University of Manchester, Oxford Road, Manchester, UK M13 9PL.
c
Chemistry Department, Faculty of Science, Al-Azhar University, 11884, Cairo, Egypt.
b
Abstract:
1,4,7,10-tetraazacyclododecane-1-carbaldehyde, is oxidized by hexacyano ferrate (III) in alkaline media to
1,4,7,10-tetraazacyclododecane-1-formic acid. The reaction is first order with respect to to aldehyde , alkaline
catalyst and hexaeyanoferrate(III). The kinetic data suggest that the oxidation involves the formation of an anion
of the substrate undergoes oxidation with hexaeyanoferrate(III) via charge transfer process. The free radical thus
produced is further oxidised to form the final products. The derived rate law is
Effect of temperature was also studied for calculating different activation parameters.
Keywords: Catalyzed oxidation, kinetics, Hexacyanoferrate, Activation parameters, Charge transfer,
thermodynamic parameters.
I.
Introduction
N-functionalized tetraazamacrocycles such as
1,4,7,10-tetraazacyclododecane (L) (Fig. 1) and
cyclam have received a great deal of attention over
the last years , because of their coordination
chemistry are used heavily in the fields of medical
imaging, radiotherapy, antitumor drugs and antiHIVagents.1-3 Moreover, the magnetic properties of
metals compounds of cyclens bearing carboxylic
acids and alcohols pendant arms have been showed a
considerable interest in magnetic interactions
occurring in these systems.4-5
Hexacyanoferrate(III) has been extensively
used to oxidize number of organic and inorganic
compounds in alkaline media.6-18 However, the
oxidation of organic substrates proceeds through
outer sphere mechanism, with the electron transfer
occurring from the reductant to the central atom via a
cyanide ligand.18
The alkaline hexacyanoferrate(III) ion acts
as an electron-abstracting reagent in redox reactions.
A free-radicals intermediate are produced from the
oxidation of some substrates 19, while oxidation of
others does not involve the production of such
radicals.20-21
Herein, we report the oxidation of 1,4,7,10tetraazacyclododecane-1-carbaldehyde (LCHO)
(Fig. 1) with hexacyanoferrate(III) in alkaline media
to
investigate
the
redox
chemistry
of
hexacyanoferrate(III), and to obtain additional
information about the thermodynamic and reactivity
of aldehyde parameter.
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O
NH HN
N H HN
(L)
NH
N H HN
N
H
(LCHO)
Figure 1: chemical structures of 1,4,7,10tetraazacyclododecane and 1,4,7,10tetraazacyclododecane-1-carbaldehyde.
II.
Materials and Methods
1,4,7,10-tetraazacyclododecane-1carbaldehyde was prepared and characterized as
reported previously.22 All chemicals used were of
reagent grade, and doubly distilled water was used
throughout the work. A solution of (LCHO) was
prepared by dissolving an appropriate amount of
Ligand in doubly distilled water. The required
concentration of (LCHO) was prepared from its
stock solution. A solution of Fe(CN)6−3 was prepared
by dissolving K3Fe(CN)6 in H2O and standardized
iodometrically. 23 NaOH and NaNO3 were employed
to maintain the required alkalinity and ionic strength,
respectively, in reaction solutions.
III.
Kinetic measurements
All kinetic measurements were performed
under pseudo first-order conditions where [LCHO]
was always in large excess over [Fe(CN)6−3 ]. The
reaction was initiated by mixing the thermostatted
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2.
Reda S. Abdel Hameed et al. Int. Journal of Engineering Research and Application
ISSN : 2248-9622, Vol. 3, Issue 5, Sep-Oct 2013, pp.767-774
solutions of Fe(CN)6−3 and LCHO, which also
contained the required concentration of NaOH and
NaNO3. The progress of the reaction was monitored
by measuring the decrease in the absorbance of
Fe(CN)6−3 in the reaction solution at 420 nm vs. time,
using a Unicam UV/VIS spectrophotometer with 1.0
cm cell. The isolation method was used in this study
where the concentration of the substance to be
monitored is changed with the others kept constant.
Aqueous solutions are prepared by making a constant
total volume for all runs.
The pseudo first order rate constants in each case
were obtained from the plots of the ln(A∞ -At) versus
time t were made (where A∞ and At are the
absorbances at infinity and at time t respectively),
which were linear to about 70% at various reactant
concentrations. The temperature was kept constant at
35º ± 1ºC, [OH-] = 0.04 mol dm-3 and the ionic
strength was maintained constant at 0.5 mol.dm-3
(NaNO3). A typical pseudo-first order plot is
presented in (Fig. 2).
0.36
0.34
Ln(A-At)
0.32
0.30
Determination of stoichiometric factors:
A reaction mixture, containing known
excess of hexacyanoferrate(III) over 1,4,7,10tetraazacyclododecane-1-carbaldehyde, was allowed
to stand in the presence of sodium hydroxide at 30
ο
C. After the completion of reaction, the result
showed that one mole of ligand consumed two moles
of
hexacyanoferrate(III)
according
to
the
stoichiometric equation (1)
The
main
oxidation
product
1,4,7,10tetraazacyclododecane-1-formic acid was extracted
with chloroform, the solvent was removed under
reduced pressure to give a white solid. The product
was dissolved in a minimum amount of methanol and
precipitated with diethyl ether.
The crystalline powder was collected by
filtration, washed with ether (2  30 ml), and dried to
afford the LCOOH as a white powder. The product
was characterized by elemental analysis and The
recorded IR spectra of aldehyde and its oxidation
product showed a decay of the band at 1725 cm−1 of
aldehyde due to(C=O) . Typical bands assigned to
1,4,7,10-tetraazacyclododecane-1-formic
spectra
were located at 1720 cm-1 is due to the CO double
bond, the broad band centered in the range 29003250 cm-1 is caused by the presence of the OH and a
band near 1400 cm-1 comes from the CO single bond.
V.
0.28
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Reaction order
The reaction orders were determined from the slopes
of log kobs vs. logc plots by varying the
concentrations of oxidant, reductant and alkali in turn
while keeping the others constant.
0.26
0.24
0
10
20
30
40
50
60
Time(min.)
Figure (2): plot of ln(A∞ -At) vs. time for the
oxidation of (LCHO) at [HCF(III)] = 4 x 10-4 M,
[OH−] = 0.4M, [LCHO] = 4 x 10-3 M, I = 0.40 M and
35 ◦C.
IV.
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Results and Discussion
VI.
Effect of alkali
To study the effect of alkali concentration
on the rate of reaction, [OH-] was varied in the range
of 0.10 to 0.50 mol.dm−3 at constant concentrations of
HCF(III) and Ligand at a ionic strength of 0.6
mol.dm−3. The rate constants of the reaction were
found to increase with increase in [OH-] (Table 1).
Plot of log kobs versus log [OH-] was linear with a
slope of 0.89 (r = 0.998), suggesting the reaction to
be first order with respect to [OH-] (Fig. 3).
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7.
Reda S. Abdel Hameed et al. Int. Journal of Engineering Research and Application
ISSN : 2248-9622, Vol. 3, Issue 5, Sep-Oct 2013, pp.767-774
On the basis of the results probable scheme of
oxidation
of
the
hydrated
1,4,7,10tetraazacyclododecane-1-carbaldehyde(LC(OH)2)by
(LC(OH)2)
The formate ion is much more stable towards
oxidation due to its mesomeric structure.
Now assuming steady state condition for the
concentration of (LCO(OH))-, the rate law might be
derived as follows:
Now keeping the experimental conditions in mind
and solving equ.(3) the final rate equation will be:
To the first approximation, if we assume that k-1[H20
] [Fe(CN)6]3-, the rate law (4) reduces to (5):
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hexacyanoferrate(III) ) ion via the formation of anion
(LCO(OH))-as an intermediate might be considered
as follows be:
(LCO(OH))-
According to equation (5), the rate law clearly
explains the experimental results i.e., first order
kinetics with respect to hexaeyanoferrate(III),
1,4,7,10-tetraazacyclododecane-1-carbaldehyde and
hydroxide ion concentration.
These results suggest that the reaction
proceeds via a charge transfer process as it has
already been observed in the course of the oxidation
of acetone, ethylmethylketone and formaldehyde by
aqueous alkaline hexacyanoferrate(III). 29- 31
Furthermore, the experimental values of ΔS* and ΔG*
were both favourable for electron transfer processes.
The high negative values of ΔS* indicates that the
intermediate complex is more ordered than the
reactants due to loss of degree of freedom. 32
XII.
Conclusion
The main product of the reaction were found
to be 1,4,7,10-tetraazacyclododecane-1-formic acid.
The reaction is first order with respect to aldahyde ,
alkali and hexaeyanoferrate(III).
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